Hung et al., “Recent progress of molecular organic electroluminescent materials and devices”, Materials Science and Engineering, R 39 (2002), 143-222 disclose that bilayer cathodes for OLEDs based e.g. on a thin (0.1-1.0 nm) LiF layer between an aluminium cathode and an aluminium quinolate electron transport layer exhibit significantly improved I-V characteristics and EL efficiencies. They explain that in OLEDs, the majority carriers are holes owing to their higher mobility and smaller injection barrier. Therefore, lowering the barrier height to electron injection is especially important as it leads to a better balance of electron and hole currents and results in a dramatic increase in luminance at a fixed bias voltage. The replacement of LiF with CsF or alkaline earth fluorides is also discussed.
U.S. Pat. No. 6,885,149 (Parthasarathy et al., Princeton University) discloses that during fabrication of an OLED, an organic electron injection layer may be doped with a metal either by depositing an organic electron injection layer on an ultra-thin layer of lithium or by depositing an ultra-thin layer of lithium on an organic electron injection layer, the organic material being e.g. 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP or bathocuproine). Use of a metal doped electron injection layer is also disclosed in U.S. Pat. No. 7,114,638, the organic component of said layer being e.g. the compound shown below:

US-A-20060040139 (Herron et al., Du Pont) discloses the use of metal Schiff base complexes in double heterostructure OLEDs as host material in the electroluminescent layer or in the electron transport layer. The complexes are based on aluminium, scandium, yttrium or a rare earth metal and the Schiff base ligand is bivalent and is e.g. of formula:
However, the use of metal Schiff base complexes as electron injection materials for OLEDs is neither disclosed nor suggested by Herron et al.